comment:

Extramedullary hematopoiesis (EMH) is commonly observed in rodents as a normal component of the splenic red pulp. It occurs more frequently in young than in aged animals, in females than in males, and in mice than in rats. Hematopoietic cell numbers may increase above normal background (
Figure 1
,
Figure 2
,
Figure 3
, and
Figure 4
) due to a variety of conditions, such as hematotoxic insult, systemic anemia, infection, hemorrhage, and neoplasia. EMH may include increased numbers of erythroid precursors (
Figure 2
and
Figure 4
, arrows), myeloid precursors (
Figure 2
and
Figure 4
, arrowheads), megakaryocytes (
Figure 2
and
Figure 4
), or various combinations of all three cell types. Erythroid precursor cells may predominate secondary to hemorrhage or erythrocyte destruction (i.e., hemolytic anemia or autoimmune-mediated anemia), whereas myeloid precursor cells may predominate secondary to inflammatory, neoplastic, or immune-mediated conditions. Plasma cell hyperplasia may accompany EMH within the red pulp. EMH is generally distinguished from neoplasia by its distribution and mixture of hematopoietic cell types showing various degrees of normal differentiation; however, severe myeloid cell proliferation may resemble granulocytic leukemia histologically. Previous terms include "hematopoietic cell proliferation," "myeloid hyperplasia," and "erythroid hyperplasia."

recommendation:

Diagnose and grade extramedullary hematopoiesis only when it is increased or decreased (though this is rare) relative to the normal background level in the concurrent controls. The diagnosis of extramedullary hematopoiesis should be modified as increased or decreased (i.e., Spleen - Extramedullary hematopoiesis, Increased or Spleen - Extramedullary hematopoiesis, Decreased). Careful comparison with concurrent controls is necessary. If one cell type predominates (i.e., myeloid, erythroid, megakaryocytic), this can be discussed in the pathology narrative. If EMH is increased secondary to neoplasia, it need not be recorded but should be described in the pathology narrative.